Articular cartilage, found at the ends of articulating bone in the body, is typically composed of hyaline cartilage, which has many unique properties that allow it to function effectively as a smooth and lubricious load-bearing surface. However, when injured, hyaline cartilage cells are not typically replaced by new hyaline cartilage cells. Healing is dependent upon the occurrence of bleeding from the underlying bone and formation of scar or reparative cartilage called fibrocartilage. While similar, fibrocartilage does not possess the same unique aspects of native hyaline cartilage and tends to be far less durable.
Hyaline cartilage problems, particularly in knee and hip joints, are generally caused by disease such as occurs with rheumatoid arthritis or wear and tear (osteoarthritis). Hyaline cartilage problems may also be the result of an injury, either acute (sudden) or recurrent and chronic (ongoing). Such cartilage disease or deterioration can compromise the articular surface causing pain and further deterioration of joint function. As a result, various methods have been developed to treat and repair damaged or destroyed articular cartilage.
For smaller defects, traditional options for this type of problem include non-operative therapies (e.g., oral medication or medication by injection into the joint), or performing a surgical procedure called abrasion arthroplasty or abrasion chondralplasty. The principle behind this procedure is to attempt to stimulate natural healing. At the defect site, the bone surface is abraded, removing approximately 1 mm. or less using a high-speed rotary burr or shaving device. This creates an exposed subchondral bone bed that will bleed and will initiate a fibrocartilage healing response. Although this procedure has been widely used over the past two decades and can provide good short term results, (1-3 years), the resulting fibrocartilage surface is seldom able to support long-term weight bearing, particularly in high-activity patients, and is prone to wear.
Another procedure, referred to as the “microfracture” technique, incorporates similar concepts of creating exposed subchondral bone. During the procedure, the cartilage layer of the chondral defect is removed. Several pathways or “microfractures” are created to the subchondral bleeding bone bed by impacting a metal pick or surgical awl at a minimum number of locations within the lesion. By establishing bleeding in the lesion and by creating a pathway to the subchondral bone, a fibrocartilage healing response is initiated, forming a replacement surface. Results for this technique are generally similar to abrasion chondralplasty.
Another known option to treat damaged articular cartilage is a cartilage transplant, referred to as a Mosaicplasty or osteoarticular transfer system (OATS) technique. This technique involves using a series of dowel cutting instruments to harvest a plug of articular cartilage and subchondral bone from a donor site, which can then be implanted into a core made into the defect site. By repeating this process, transferring a series of plugs, and by placing them in close proximity to one another, in mosaic-like fashion, a new grafted hyaline cartilage surface can be established. The result is a hyaline-like surface interposed with a fibrocartilage healing response between each graft.
Such an OATS procedure is technically difficult, as all grafts must be taken with the axis of the harvesting coring drill being kept perpendicular to the articular surface at the point of harvest. Also, all graft placement sites must be drilled with the axis of a similar coring tool being kept perpendicular to the articular surface at the point of implantation. Further, all grafts must be placed so that the articular surface portion of these cartilage and bone plugs is delivered to the implantation site and seated at the same level as the surrounding articular surface. If these plugs are not properly placed in relation to the surrounding articular surface, the procedure can have a very detrimental effect on the mating articular surface. If the plugs are placed too far below the level of the surrounding articular surface, no benefit from the procedure will be gained. Further, based on the requirement of perpendicularity on all harvesting and placement sites, the procedure requires many access and approach angles that typically require an open field surgical procedure. Finally, this procedure requires a lengthy post-operative non-weight bearing course.
Transplantation of previously harvested hyaline cartilage cells from the same patient has been utilized in recent years. After the cartilage is removed or harvested, it is cultured in the lab to obtain an increase in the number of cells. These cells are later injected back into the focal defect site and retained by sewing a patch of periosteal tissue over the top of the defect to contain the cells while they heal and mature. The disadvantages of this procedure are its enormous expense, technical complexity, and the need for an open knee surgery. Further, this technique is still considered somewhat experimental and long-term results are unknown. Some early studies have concluded that this approach offers no significant improvement in outcomes over traditional abrasion and microfracture techniques.